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Team Comment on OWL 1.1 Web Ontology Language Submission

W3C is pleased to receive the OWL1-1 submission from SRI International, TopQuadrant, University of Manchester and webMethods, Inc., including expressions of support from a number of other W3C member organizations. We are grateful to the submitters for collecting the experience of a significant community of OWL users and developers, especially the OWL: Experiences and Directions workshops in Galway, Ireland ( 11-12 November, 2005 ) and Athens, Georgia, USA (10-11 November, 2006) and contributing the resulting proposal to W3C.

OWL 1.1 extends the W3C OWL Web Ontology Language with ... additional property and qualified cardinality constructors, extended datatype support, simple metamodelling, and extended annotations.

The W3C OWL Recommendation of 2004 includes an RDF-Compatible Model-Theoretic Semantics of OWL and an OWL Full class of documents. This OWL 1.1 specification does not extend the RDF-compatible semantics for of OWL to cover the new features of OWL 1.1. Complete reasoning in OWL Full is intractable (it is estimated to be as hard as first-order logic, where there is provably no decision procedure for determining for an arbitrary formula P, whether P is valid). Complete reasoning in OWL DL and OWL 1.1 is tractable, but the expressivity is limited; the domain of discourse is constrained so that individuals, classes, and properties are separated. OWL 1.1 extends the expressivity of OWL and its RDF/XML syntax is less constrained than OWL DL, though still subject to a regularity syntactic restriction (see section 7 Nonstructural Restrictions on Axioms of the Functional Syntax).

The OWL 1.1 Tractable Fragments document explores the trade-off between expressivity and performance in various subsets of OWL 1.1. It includes a concise diagram. Note the distinction between "RDFS" "RDF Schema" as used in this document versus the usage in the W3C Recommendation RDF Vocabulary Description Language 1.0: RDF Schema, where individuals, classes, and properties are all part of the same domain of discourse. The following diagram clarifies the relationship between the two uses of "RDFS":

OWL fragments, including RDF Schema and OWL Full

The syntax of OWL 1.1 is specified with UML diagrams and a BNF. The BNF-based textual syntax seems to work well as a specification mechanism, but if it is to be used concretely for ontology storage and transmission in the Web, details such as how to include quote characters in strings should be clarified, and limitations such as the omission of relative URI references (which are allowed in the XML and RDF/XML syntax) should be lifted.

Section 3. Ontologies of the Functional Syntax document says Each ontology is uniquely identified with an ontology URI, which is entirely straightforward; however, it goes on to say This URI need not be equal to the physical location of the ontology file. To make the "physical location of the ontology file" different in such away that the ontology is not available as a representation goes against the Available representation best practice of Architecture of the World Wide Web, Volume One.

Next Steps

The submitters suggest that W3C consider this as a starting point for work in a new working group. W3C welcomes discussion of this suggestion in the public-owl-dev mailing list. We look forward to discussion of:

Dan Connolly, Feb 2007
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